Apparatus and method for electrochemical analysis



, June 24, 1941. L, A, MATHESON ETAL 2,246,981-

APPARATUS AAND METHOD FOR ELGTROGHEMIGAL. ANALYSIS Filed Aug. 27, 1938 5 sheets-sheet 1 June 24,- 1941 L. A. MATHEsoN ET AL 27,246,981

APPARATUS AND METHOD FOR ELCTROCHEMICAL ANALYs/s Filed Aug. 27, 1938 3 Sheets-Sheet 2 June 24, 1941 L. A. MATHEsoN Erm. 2,246,981y

APPARATUS AND METHOD FOR ELCTROCHEMICAL ANALYSIS Filed Aug.. 27, 1958 5 sheets-sheet 5 Patented June 24, 1941 APPARATUS AND METHOD FOR ELECTRO- CHEMICAL ANALYSIS Lorne A. Matheson, Nathaniel B. Nichols, Harold A. Robinson and Clyde F. Du Chene, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Michigall Application August 27, 1938, Serial No. 227,088

13 Claims.

The present invention relates to an apparatus and method for the study of electrochemical phenomena. More particularly it concernsv apparatus for the electrochemical analysis of solutions.

It has long been known that every electrolytic deposition takes place at a definite voltage, termed the deposition potential, which is characteristic of the particular substance being deposited. If a voltage less than the deposition potential of an electrolyte is applied between electrodes immersed in a solution of that substance, no deposition takes place and substantially no current iiows. On the other hand, if a voltage at least equal to the deposition potential is applied, electrolysis occurs and the strength of the electrolyzing current is proportional to the concentration of the electrolyte in the solution. It follows, therefore, that when a gradually increasing voltage is applied between electrodes immersed in a solution of several electrolytes, no appreciable current will pass until the lowest deposition potential is reached. When this value is attained, a current proportional to the concentration of the respective "substance ows; when the next higher deposition potential is reached, a sudden increase in current occurs proportional to the"concentration of this second substance, and so on. 'I'he determination of such a current-voltage curve thus amounts to a qualitative and quantitative analysis of a solution.

The theory and practice of this analytical method, frequently termed polarographic analysis, have been widely investigated. (See The Polarographic Method, by J. Heyrovsky, in Physikalische Methode in der Chemischen Analyse, Vol. II, by W. Bttger, Akad-Verlags- Ges., Leipzig, 1936, or the extensively annotated monograph Chemische Analysen mit dem Polarographen by H. Hohn, J. Springer, Berlin, 1937.) Analyses are for the most -part carried out with the aid of special apparatus, particularly the HeyroVsky-Shikata Polarograph (Rec. trav. chim. 44, 496-8 (1925); The Polarograph, E. Leybolds Nachfolger A.G., Kln, (1937)), and tne current voltage curve drawn by this instrument is generally referred to as a polarogram In the usual polarograph, a reference electrode of known potential and an indicating electrode are dipped into a vessel containing the solution to be analyzed. A slowly increasing potential is applied between the electrodes by means of a slide-wire potentiometer arranged so that the slide is drawn at a lmiform rate by asmall motor. The variations in current caused by the increasing voltage are indicated by a mirror galvanometer in series with the cell. For recording, a lamp casts a fine pencil of light onto the mirror of the galvanometer, from which it is reected to a strip of photographic paper Wound on a rotatable drum.

Despite its obvious utility, the polarographc method of analysis has not met with widespread acceptance because of certain instrumental diiiculties inherent in the polarograph Thus, the apparatus requires from 3 to 10 minutes to draw a complete po1arogram, so that the analyses of solutions which are changing rapidly in composition cannot be made. Moreover, the polarogiam is recorded photographically, with the result that the slowness and inconvenience characteristic of any procedure requiring the development of photographs are encountered. In addition, the apparatus is appreciably affected by vibration. These and'other disadvantages have seriously limited the application ofv the polarograph to electrochemical analysis, and have rendered it largely useless in titration analyses and in the study of rapid electrolytic processes.

An object of the present invention is'to provide an apparatus for the study of electrodeposition phenomena and for the analysis of solutions, which utilizes in part the general principles of polarographic analysis referred to above, but which does not possess the disadvantages inherent in the polarograph. Another object is to provide an instrument for the instantaneous determination and continuous visual indication of current-voltage relations in electrolytic systems.

We attain these objects by apparatus adapted to applyy a rapidly increasing voltage Iacross an indicatingelectrode and a reference electrode immersed in a solution and to indicate the variations in current flowing between the electrodes. The invention, then, consists in the arrangement of apparatus hereinafter described, the annexed drawings and following description setting forth in detail but several of the ways in which the -principle of the invention may be employed. In the drawings, in which like elements are numbered alike throughout:

Fig. 1 is a diagrammatic representation of a preferred form of the apparatus.

Fig. 2 is a current-voltage diagram observed with the apparatus illustrated in Fig. 1, using a slow mercury dropping rate.

Fig. 3 is a current-voltage diagram observed Yproximately vsinusoidally,

with the apparatus illustrated` in Fig. 1 using a synchronized mercury dropping rate.

Fig. 4 is a diagrammatic representation of a modified form of the apparatus.

Fig. 5 is a diagrammatic representation of another modified form of the apparatus.

Fig. 6 is an oscillogram showing the effect of a high series resistance on a record obtained with the apparatus illustrated in Fig. 4.

Fig. 7 is an oscillogram showing the effect of a low series resistance on a record obtained with the apparatus illustrated in Fig. 4.-

Fig. 8 is a diagrammatic representation of a bridge circuit utilizing the apparatus illustrated in Fig. 4.

Figs. 9, 10, and 11 are representative currentl voltage diagrams observed with the apparatus illustrated in Fig. 8. v

Referring particularly to Fig. 1, the solution to be analyzed is placed in a suitable vessel II containing a pool of mercury I2. Dipping into the vessel is a dropping mercury electrode I3 consisting of a reservoir I4, a capillary tube I5, and a ne tip I6 from which the mercury falls continually in the form of minute droplets; this electrode acts as an indicating cathode. .The mercury pool I2, which has an area large in comparison to that of the cathode, serves as a reference anode. As a voltage source, an alternator I1 is connected through a variable transformer I8 and a lead I9 in series with a direct c'urrent source consisting of a potentiometer 20 operated by a battery 2|. A lead 22 connects the potentiometer with the anode I2, and leads 23 and 25 connect the transformer I8 with the mercury cathode I3 through a resistance 24.

The potential applied between the electrodes I2 and I3 is transmitted by leads-25 and 26 to a vacuum tube amplifier 21, and the amplified voltage is impressed by way of leads 28 and 29 on the horizontal deflection plates 301-30 of a standard cathode ray oscillograph 3l. Similarly the potential across the terminals of the resistance 24 is transmitted through an amplifier 32 and leads 33 and 34 to the vertical deiiection plates 35--35 of the oscillograph 3l.

In the apparatus illustrated, the cathode ray oscillograph 3l is of the four-plate electrostatic deflection type with a sensitivity ofabout '10 volts per inch and preferably with a screen diameter of at least five inches. The amplifiers 21 and 32 should have a suitable gain and should preferably transmit the Wave form of the applied voltage accurately, i. e. the amplification should be uniform for all frequencies concerned and the phase shift should be negligible. Various satisfactory amplifiers have been described in the art (Electronics, 10, No. 6, p. 26-2'7, 1937).

In conducting an analysis with the apparatus shown in Fig. 1, the alternator I1 and transformer I8 are adjusted to produce an A. C. voltage of the desired magnitude and frequency. An A. C. voltage of about 0.9 volt (root mean square) is suitable and a frequency of not less than about one cycle per second is preferably employed, 60 cycles being convenient. The potentiometer 20 is then adjusted to bias this alternating voltage to an extent suiicient that the resulting voltage, when applied across the cell, causes the potential at the dropping mercury cathode I3 to vary'apfor example, from about 0.0 to about 2.5 volts negative with respect to the reference anode I2. Y, It will be appreciated that for certain types of analyses the magof the voltage bias may be changed from the values just mentioned, and that the electrodes may even alternate in polarity. However, for most analyses it is desirable that the electrodes be at about the same potential at the minimum in' each voltage cycle, and that the maximum voltage in each cycle be in excess of the deposition potentials of all substances in solution.

In operation, the horizontal deflection of the cathode ray oscillograph 3I is proportional to the voltage across the electrodes I2 and I3, and the vertical deflection is proportional to the voltage drop across the resistance 24 and hence is proportional to the current flowing through the cell. Accordingly, for each cyclic variation in the voltage applied across the cell, there is traced upon the fluorescent screen of the oscillograph a complete current-voltage curve for the solution in the cell. This curve resembles a Heyrovsky polarogram, and, as explained above, is characteristic of the electrolytically reducible materials in solution. Moreover, in our instrument a complete curve is traced with great rapidity, e. g. 60 times a second, instead of once in 3 to 10 minutes, as on the po1arograph.

A typical current-voltage oscillogram observed with our apparatus is represented in Fig. 2. This pattern was obtained with an aqueous solution containing two reducible substances, the mercury cathode being adjusted so that a drop formed once a second. At such dropping rate, when using a 60-cycle voltage variation, a series of sixty current-voltage curves is traced for each individual mercury drop, the first few' traces A, B, C on a single newly-formed drop being illustrated. It is evident that as the drop grows in size, i. e. as the cathode area increases, the current (vertical displacement) in each successive trace also increases. However, in any single trace, the voltages (horizontal displacements) at which rapid increases or breaks in the current occur, as at 36 and 31 on trace A, are indicative of the particular reducible substances in solution, and the height of these breaks is proportional to the concentration of said substances.

y In practice the horizontal and vertical scales on nltude of the voltage variation and the extent the oscillograph screen are first calibrated in terms of cell voltage Vand current, respectively, and the oscillograms are then interpreted in a manner similarto the Heyrovsky polarograms. For most analyses a visual observation of the oscillograms is sufficient, but if desired a permanent photographic record can be made. When photographs are taken, it is not essential that a cyclically varying voltage be applied across the cell, but a single rapid voltage sweep may be sufiicient, e. g. one requiring not over about one second to complete.

When operating as described above, several current-voltage curves are traced upon the oscillograph screen for each individual mercury drop. Under these conditions successive current-volt- ,age traces are not superimposed, but tend to shift somewhat as the mercury drop grows, as was illustrated in Fig. 2. In many instances this continuous shifting of the oscillograph pattern is not disadvantageous, but for accurate analytical work it is desirable that the pattern be'I stationary, i.. e. that successive traces on the oscillograph screen be superimposed exactly. We have found that this result may be attained by synchronizing the mercury dropping rate with the cyclic variation of the voltage in a manner hereinafter explained so that one drop of mercury falls for each voltage sweep applied to the cell. With such synchronized dropping rate, all electrical conditions influencing the oscillograph pattern, such as mercury drop size, etc., are identical for each successive voltage sweep and the current-voltage traces are accurately superimposed, giving a stationary oscillogram. Fig. 3 represents such an oscillogram observed with a synchronized dropping mercury electrode, using the same solution as that employed in obtaining Fig. 2. A comparison of Fig. 3 with Fig. 2 clearly shows the advantages of drop synchronization.

The mercury dropping rate may be synchronized with the cyclic variation of the voltage by purely mechanical means. However, we have found that such synchronization may most conveniently be accomplished by taking advantage of a phenomenon apparently based on .the known fact that the interfacial tension between mercury and a solution is a function of the electrical potential applied across the mercury-solution interface. Reference to the electro-capillary curves for mercury (see Principles of Experimental and Theoretical Electrochemistry, by M. Dole, `pages 467-470, McGraw-Hill Co., New York, 1935) shows'that4 the interfacial tension falls nearly to zero when a negative voltage of at least 1.3-2.0 volts is impressed between the mercury and the solution. In synchronizing an electrode,

e. g. in the apparatus illustrated in Fig. 1, the

mercury level in the dropping electrode I3 is first adjusted so that, when no potential is applied, the hydrostatic pressure of the mercury column only slightly exceeds the force due to the interfacial tension of the mercury clinging to the tip, and mercury drops form at a slow rate. Then, When a negative voltage increase of at least 1.3 volts is applied to Ithe electrode, such change in potential immediately reduces the interfacial tension of the mercury to a point where it is no longer approximately sufficient to support the column of mercury in the electrode, and a drop instantly falls from the tip. This series of events is repeated for each voltage increase applied to the electrode. That is, in our apparatus Ithe mercury electrode may be made to drop in synchronism with the cyclically varying voltage by adjusting the mercury level in the electrode and by controlling the magnitude of the varying voltage so that the maximum potential applied across the electrodes I2 and I3 exceeds 1.3 volts.

For example, an analysis was carried out at 30 C. with a solution containing zinc chloride (0.001 normal) and lithium chloride (0.10 normal). The capillary tip I6 .of the dropping mercury electrode had a diameter of 0.060 m. m. The resistance 24 in series with the cell was 1000 ohms. A voltage having a maximum value of 2.2 volts and a frequency of 5.7 cycles per second was applied across the electrodes. Then, by adjusting the level of the mercury in :the electrode to a height of 445 m. m. above the capillary tip, the mercury was caused to drop in synchronism with the cyclic variations of the. voltage. Under such condition-s, the mercury i'low was 0.65 gram per minute.

It has been mentioned that with theapparatus shown in Fig. l, the voltage applied between the electrodes I2 and I3 varies sinusoidally with For many analyses, however, it is prefertime. able to employ a voltage which varies linearly the apparatus illustrated in Fig. 4, in which the arrangement of the cathode-ray oscillograph 3|, amplifiers 2'I and 32, electrodes I2 and I3, impedance 24, and potentiometer 20 is identical in all respects with that shown in Fig. 1. The recurrent linear voltage sweep is yproduced in this case by a circuit comprising in parallel a rotating make-and-break contactor 38, a condenser 39, and a battery 40 in series with a resistance 4I. When the contactor 38 opens, the battery 40 charges the condenser 39 at an essentialy uniform rate, thereby causing the voltage across the electrodes I2 and I3 to increaseapproximately linearly with time. Then when'the contactor 38 closes, the condenser 39 is shorted, and the electrode voltage -returns almost instantly to that of the potentiometer 20. The frequency of the recurrent voltage sweep maybe controlled by changing the frequency with which the contactor 38, shorts the condenser 39, i. e. by changing the speed of rotation. For sweep frequencies up to about 30 cycles 'a simple mechanical contactor is satisfactory, but for higher frequencies other types of contactors, e. g. an electronic shorter, may be employed. For most analystical Work a frequency of 5 cycles per second has been found convenient. With such frequency the condenser 39 may be a 200 mfd. capacitor, the battery 60 a 45-vo1t dry cell, and the resistance 4I a 20,000 ohm resistor.

In the apparatus illustrated in Fig. 4, a standard calomel half-cell 42 is used as the voltage reference electrode. In this instance the horizontal deflection of the oscillograph 3I is responsive to the variations in potential of the indicating electrode I3 relative to the standard half-cell 42. Because the potential of a calomel electrode is known with great accuracy, the electrode arrangement shown in Fig. 4 is advantageous for analyses where precision of .the voltage measurements is important.

Indicating electrodes other than the dropping mercury cathode may be employed in our instrument. Thus Fig. 5 illustrates an arrangement of apparatus using a platinum electrode as an indicating anode for the study`of oxidation phenomena. Referring to the drawings, the solution to be analyzed is placed in a vessel II containing a pool of mercury I2 which serves as a reference cathode, and a i'lne platinum wire 43 is dipped into `the solution. A recurrent substantially linear variation is applied between the electrodes I2 and 43 by means of the current comprising a contactor 38, condenser 39, and battery 40, as hereinbefore explained. 'Ihe current flowing through the cell is indicated by the vertical deflection of the cathode-ray oscillograph 3|. In the circuit illustrated in Fig. 5 the necessary horizontal deflection of the oscillograph is supplied by a standard oscillographic sweep circuit 44 I which may, if desired, be synchronized in known with time, i. e. a recurrent saw-tooth voltage sweep which increases uniformly, for example from 0.0 to 2.5 volts, and then returns very rapidly to zero. This result may be attained with manner with the varying voltage applied to the cell. Alternatively, the horizontal deflection of the oscillograph may be controlled by the varying electrode voltage itself, as shown in Figs. 1 and 4.

It will, of course, be appreciated that the sensitivity of our new apparatus may be varied at will within wide limits by changing the gain of the oscillograph ampliers 21 and 32. Moreover, in practice We have found that the ease of interpreting theoscillograms may further be varied somewhat by changing the magnitude of the resistance 24 in series with the cell. The eifect of varying this resistance is to change the form be proportional, mathematically speaking,

of the current-voltage oscillogram without markedly changing either-` the position in volts or height 'in amperes of the breaks in the curvev corresponding to the various substances in the solution being analyzed. The result of such resistance changes may readily be seen by a comparlson of Figs. 6 and '7. Both figures represent portions of osclllograms observed with an aqueous solution containing cadmium chloride (0.0008 normal), zinc chloride (0.0008 normal), manganous chloride (0.0008V normal), and lithium chloride (0.10 normal) using a synchronized dropping mercury cathode in the apparatus shown in Fig. 4. Fig. 6 was obtained with a resistance of 100,000 ohms in series with the cell, and Fig. 'l was taken using a 60 ohm resistance. Both curves show the characteristic breaks corresponding to the deposition potentials of the cadmium, zinc, manganese, and lithium ions. However, the high-resistance curve (Fig. 6) consists of a series of steps or plateaus, whereas the low-resistance diagram (Fig. '7) exhibits distinct peaks. It will be appreciated that the ease of interpreting these two types of oscillograms ls different, and that each type possesses its peculiar utility.

In certain analyses it may be desirable to replace the resistance 24 in series with the electrolytic cell by an inductance or other impedance. For example, li an lnductance bridge be substituted andsuitably connected in known manner, the vertical deflection of Vthe oscillograph will to the iirst derivative of the current with respect to time, rather than to the current itself. This and other minor variations in our apparatus will doubtless occur to one skilled in the art, and are to be considered within the scope of the invention.

In the forms hereinbefore described, out instrument is well suited to the rapid analysis of electrolytlc solutions, and to the study of electrodeposition and electroreduction phenomena in general. For certain purposes, however, particularly for volumetric analysis, the utility of the apparatus may be further extended by adapting the same to a suitable network in the nature of a Wheatstone bridge, utilizing. two identical indicating electrodes. Such a circuit, using two dropping mercury cathodes, is illustrated in Fig. 8. Referring to the drawings, the two branches of the bridge consist, respectively, of the leads 25 and 25', electrodes I3 and I3', reference anodes I2 and I2', junctions 45 and 45', and variable lmpedances 46 and .46. sweep may beimpressed across the bridge circuit by a generating circuit comprising a contactor 38, a condenser 39, and a battery 40, as hereinbefore described. This voltage sweep, as lt appears across the bridge arm including the electrode I3', is applied to the horizontal deflection plates -30 of the oscillograph 3l by way of leads 25 and 41 and an ampliiier 2l. The vertical deflection plates --35 are connected to the bridge junctions 45 and 45 through leads 41 and 48, a lead 49, and anl amplifier 32, and thus serve to indicate unbalance between ,the two sides of the bridge.

In a preferred manner of carrying out analyses with the bridge circuit of Fig. 8, the two vessels II and Il are filled with equal quantities of the same solution of some non-'interfering electrolyte, e. g. 0.10 normal lithium chloride. The recurrent voltage sweep is then applied across the cells and the electrodes I3 and I3' are adjusted to A recurrent voltage drop in synchronism with the voltage impulses. The impedances 4l and 48 are then varied until the two halves of the bridge are in balance, l. e. so that no potentialv exists between the junctions and 45'. This state of balance is indicated by the appearance on the screen of the oscillograph of a horizontal line at a position corresponding to zero vertical deiiection, as shown in Fig. 9. When the bridge is thus balanced, a measured quantity of the solution to be analyzed is added to the electrolyte in the vessel-"I I. vThe presence of reducible substances in the unknown solution so added causes an -unbalance of the bridge circuit during each recurrent voltage sweep, such unbalance occurring only at the voltages corresponding to the deposition potentials of the unknown substances. This unbalance is indicated by the appearance of one or more peaks on the cathode-ray osclllogram and illustrated ln Fig. 10 as peak 50. As explained above, the potentials at which these peaks occur are indication Aof the nature of the unknown materials, and the heights of the peaks are a measure of the concentration of such substances.

For more accurate work, the identity oi the unknown added to vessel I I may be further verified by adding a quantity of the substance suspected of being the unknown to the other electrode vessel II. If the substance so addedis identical with the unknown material, the bridge 'circuit will tend more nearly to balance and the height of the oscillogram peak will be reduced. However, if the suspected material added to vessel II' is not the unknown, the bridge will be further unbalanced, and another peak of opposite direction, as illustrated in Fig. 11 as peak 5I, will appear on the oscillogram.

When the identity of the unknown material in vessel Il, has been established, its concentration may be determined by adding measured quantities of a standard solution of the same material to the vessel Il'. When the concentration of the material so added becomes equal to that of the unknown material in vessel Il, the bridge circuit returns-to balance and the oscillogram again becomes a straight line, as in Fig. 9. It is evident, therefore, that the bridge circuit illustrated in Fig. 8 is well adapted to volumetric analyses, the endpoint being indicated by a straight line oscillogram. For example, with suitable sensitivity, the apparatus may be used for a rapid approximate volumetric estimation of sodium in the presence of potassium, and vice versa, a result practically unobtainable with known analytical methods.

Our new apparatus may also be applied to the study of rapid electrode processes other than those described, e. g. the mixing of solutions, diffusion and adsorption phenomena, rates of reaction, etc., in both aqueous and non-aqueous solutions. The arrangements of apparatus hereinbefore described represent preferred embodiments of our invention only, and it is to be understood that the invention is not limited thereto, but is coextensive in scope with the appended claims.

We claim:

1. An electrochemical apparatus comprising means for containing a solution to be analyzed, a reference electrode and an indicating electrode disposed to contact said solution, means for impressing a recurrent substantially linear voltage sweep across said electrodes giving rise to a varying flow of current therebetween, and indcating means immediately responsive to the variations in'said current.

2. An electrochemical apparatus comprising means for containing a solution to be analyzed,

a reference anode and a droppingmercury cathode disposed to contact said solution, means for impressing a cycllcally varying voltage having a frequency of at least one cycle per second across said electrodes giving rise to a varying flow of current therebetween, and indicating means instantaneously responsive to the variations in said current.

3. An electrochemical apparatus comprising means for containing a solution to be analyzed, a reference anode and a dropping mercury cathode disposed to contact said solution, means for impressing a recurrent substantially linearvoltage sweep across said electrodes giving rise to a varying flow of current therebetween, and a cathode-ray oscillograph adapted to indicate the variations in said current as a function of said varying voltage sweep. v

4. In an electrochemical apparatus, the circuit comprising in series a reference electrode, an indicating electrode, an impedance and a source of current adapted to produce a cycllcally varying voltage across said electrodes, and in combination therewith a cathode-ray oscillograph having one pair of deflection plates operatively connected across the said impedance, and the other pair of plates operatively connected across the said indicating and reference electrodes, whereby the variations in current flowing between the said electrodes are indicated on the oscillograph screen as a function of the lvoltage across the electrodes.

5. In an electrochemical apparatus, the circuit comprising in series a reference electrode, an indicating electrode, an impedance, a source of alternating current, and a source of direct current, and in combination therewith a cathoderay oscillograph having one pair of deflection plates operatively connected across the said impedance, whereby the variations in current ilowing between the said electrodes are indicated upon the oscillograph screen.

6. In an electrochemical apparatus, the-circuit comprising in series a reference anode, a ldropping mercury cathode, an impedance and a source of current adapted to produce a cyclically varying voltage having a frequency of at least one cycle per second across said electrodes, and in combination therewith a cathode-ray oscillograph having one pair of deflection plates operatively connected across the said impedance. whereby the variations in current flowing between the said electrodes are indicated upon the oscillograph screen.

7. An electrochemical apparatus comprising a branched circuit in the nature of a Wheatstone bridge having in each branch thereof 'a variable impedance and an electrolytic cell comprising an indicating electrode and a reference electrode. means for applying a cyclically varying voltage across said branched circuit, whereby a cycllcally varying voltage is caused to exist across each cell and 'a varying current is caused to ow therein, and means for indicating any unbalance between the current flowing in the two branches as a function of the voltage across one of the cells 8. An electrochemical apparatuscomprising a branched circuit in the nature of a Wheatstone bridge having in each branch thereof a variable impedance and an electrolytic cell comprising a reference anode and a dropping mercury cathode adapted -to be synchronized with cyclic voltage variations applied thereto, means for applying a cycllcally varying voltage across said branched circuit vwhereby a cyclic varying voltage is caused to exist across each cell and a varying'l current is caused to iiow therein, and a cathode-ray osclllograph adapted to indicate any unbalance between the current flowing in the branches as a function of the voltage across one of the cells.

9. The method of analyzing solutions which comprises applying a recurrent substantially linear voltage sweep between a reference electrode an-d an indicating electrode immersed in the solution -to be analyzed, whereby a varying current is caused to flow between said electrodes, and observing the variations in current caused by said varying voltage.

10. The method of analyzing solutions which comprises applying a voltage varying cycllcally between about 0.0 and about 2.5 volts and having a frequency of Iat least one cycle per second between a reference electrode and a dropping mercury cath-ode immersed in the solution .to be analyzed, whereby a varying current is caused t-o iiow between said electrodes, and observing the variations in current caused by said varying voltage.

1'1. The method of analyzing solutions which comprises applying a cyclic voltage variation having a frequency of at least one cycle per second between a reference anode and a dropping mercury cathode immersed in the solution to be analyzed, whereby a varying current is caused to flow between said electrodes, synchronizing the dropping rate of -said mercury cathode with the cyclic variations of the voltage, and observing the variation in current caused by said varying voltage.

12. In a method for electrochemical analysis wherein a dropping mercury electrode is employed as an indicating electrode, the step of causing said electrode to form drops at a predetermined frequency which comprises applying to said electrode a voltage varying cyclically at said predetermined frequency and having a maximum value of atleast 1.3 volts.

13. In a method for electrochemical analysis wherein a cyclic voltage variation is applied between a reference electrode and a dropping mercury cathode, thereby causing a varying current to iiow between said electrodes, the step which comprises applying` at least one complete cyclic variation of the voltage -to each individual drop formed by the dropping mercury electrode.

LORNE A. MATHESON. NATHANIEL' B. NICHOLS. HAROLD A. ROBINSON. CLYDE F.,DU CHENE. 

